windows: watch for system theme change

Requires Windows SDK version 10.0.18362.0 and Windows Runtime
10.0.10240.0.

GitLab: #723
Change-Id: I166c236bf24419b4abfe323f88e470dd07afbd4f

src/app/utilsadapter.cpp

@@ -697,6 +697,12 @@ UtilsAdapter::isSystemThemeDark()
     return false;
 #else
 #if defined(WIN32) && __has_include(<winrt/Windows.Foundation.h>)
+#if WATCHSYSTEMTHEME
+    if (!settings) {
+        settings = UISettings();
+        settings.ColorValuesChanged([this](auto&&...) { Q_EMIT appThemeChanged(); });
+    }
+#endif
     return readAppsUseLightThemeRegistry(true);
 #else
     qWarning("System theme detection is not implemented or is not supported");

src/app/utilsadapter.h

@@ -35,6 +35,15 @@
 
 #if defined(WIN32) && __has_include(<winrt/Windows.Foundation.h>)
 #include <winrt/Windows.Foundation.h>
+
+#define WATCHSYSTEMTHEME \
+    __has_include(<winrt/Windows.UI.ViewManagement.h>)
+
+#if WATCHSYSTEMTHEME
+#include <winrt/Windows.UI.ViewManagement.h>
+#endif
+
+using winrt::Windows::UI::ViewManagement::UISettings;
 #endif
 
 class QClipboard;
@@ -151,5 +160,9 @@ private:
     GSettings* settings {nullptr};
     GSettingsSchema* schema {nullptr};
 #endif
+
+#if WATCHSYSTEMTHEME
+    UISettings settings = NULL;
+#endif
 };
 Q_DECLARE_METATYPE(UtilsAdapter*)